Chapter 14: Chemical Effects of Electric Current

Science

  (www.olympiadsuccess.com)

  Chapter 14: Chemical Effects of Electric Current

  Class: VIII

   Exemplar Solutions

 

MULTIPLE CHOICE QUESTIONS

Question 1

 An electric current can produce

 (a) heating effect only.  

(b) chemical effect only.

(c) magnetic effect only. 

(d) chemical, heating, and magnetic effects.

Answer 1 (d)

Question 2

 Boojho and Paheli performed experiments taking similar bulbs and cells but two different solutions A and B as shown in Fig.14.1

They found that the bulb in the setup A glows more brightly as compared to that of the setup B. You would conclude that

(a) higher current is flowing through the circuit in setup A.

(b) higher current is flowing through the circuit in setup B.

(c) equal current is flowing through both the circuits.

(d) the current flowing through the circuits in the two setups cannot be compared in this manner.

Answer 2 (a)

Question 3

 Boojho’s uncle has set up an electroplating factory near his village. He should dispose off the waste of the factory

(a) in the nearby river.

(b) in the nearby pond.

 (c) in the nearby cornfield.

(d) according to the disposal guidelines of the local authority.

Answer 3 (d)

Question 4

 When electric current is passed through a conducting solution, there is a change of colour of the solution. This indicates

(a) the chemical effect of current.

 (b) the heating effect of current.

(c) the magnetic effect of current.

 (d) the lightning effect of current.

Answer 4 (a)

Question 5

Which one of the following solutions will not conduct electricity?

 (a) lemon juice  (c) tap water

 (b) vinegar   (d) vegetable oil

Answer 5 (d)

 

Question 6

Which of the following metals is used in electroplating to make objects appear shining?

(a) iron  (c) chromium

(b) copper  (d) aluminium

Answer 6 (c)

Question  7

Which of the following solutions will not make the bulb in Fig 14.2 glow?

 (a) sodium chlorides

 (b) copper sulphate

 (c) silver nitrate

(d) sugar solution in diluted water

Answer 7 (d)

 VERY SHORT ANSWER QUESTIONS

Question 8

 Fill in the blanks

(a) The object to be electroplated is taken as ____________ electrode.

(b) One of the most common applications of chemical effect of electric current is ______________.

(c) Small amount of a mineral salt present naturally in water makes it a______________ of electricity.

(d) Electroplating of ______________ is done on objects like water taps and cycle bell to give them a shiny appearance.

Answer 8

(a) Cathode  (c) Conductor

 (b) Electroplating   (d) Chromium

Question 9

 Why is a layer of zinc coated over iron?

Answer 9

A coating of zinc is provided to protect iron from corrosion and rust.

Question 10

 Will the solution of sugar in distilled water conduct electricity?

Answer 10

No

Question 11

 Name the effect of current responsible for the glow of the bulb in an electric circuit.

Answer 11

Heating effect of electric current.

 

SHORT ANSWER QUESTIONS

Question 12

Boojho made the circuit given in Fig. 14.3 and observed that the bulb did not glow. On Paheli’s suggestion he added one more cell in the circuit. The bulb now glows. Explain.

Answer 12

Addition of another cell increased the current through the bulb sufficiently to make it glow.

Question 13

Paheli set up an experiment using liquid A in the beaker as shown in Fig. 14.4. She observed that the bulb glows. Then she replaced the liquid A by another liquid B. This time the bulb did not glow. Boojho suggested replacing the bulb by an LED. They observed that the LED glows. Explain.

Answer 13

The current through liquid B could be weak and therefore unable to make the bulb glow. However, it was strong enough for the LED to glow.

Question 14

Paheli wants to deposit silver on an iron spoon. She took silver nitrate (AgNO3) solution in a beaker and setup a simple circuit for electroplating. Which terminal of the battery should the spoon be connected to? What material should the other electrode be made of?

Answer 14

The spoon should be connected to the negative terminal of the battery. The other electrode should be made of silver.

Question 15

 Why is tin electroplated on iron to make cans used for storing food?

Answer 15

Tin is less reactive than iron. Tin coating prevents food from coming in contact with iron and thus prevents it from getting spoiled.

Question 16

 Observe Fig. 14.5.

 Which of these two circuits A or B shows the correct observation?

Answer 16

Diagram A shows the correct observation.

Question 17

Observe the following circuits carefully. In which circuit will the bulb glow. Write ‘Yes’ or ‘No’ in the blank space provided along each of the circuit given in Fig. 14.6.

Answer 17

No, Yes, No, Yes

LONG ANSWER QUESTIONS

Question 18

 An electric current is passed through a conducting solution. List any three possible observations.

Answer 18

• Bubbles of gas may be formed on the electrodes.
• Deposits of metal may be seen on electrodes.
• Change in the colour of the solution may take place.
• The solution may get heated.(Any three)

Question 19

 In the circuit given as Fig. 14.7, Boojho observed that copper is deposited on the electrode connected to the negative terminal the battery. Paheli tried to repeat the same experiment. But she could find only one copper plate. Therefore she took a carbon rod as negative electrode. Will copper be still deposited on the carbon rod? Explain your answer.

Answer 19

Yes, copper from the copper sulphate solution will be deposited on the carbon rod. Copper from the copper plate will be dissolved into the copper sulphate solution for electroplating.

Question 20

Observe the circuit given in Fig. 14.8

Boojho set up this circuit for purification of copper. What will be the nature of – (i) plate A (ii) plate B (iii) the solution.

Explain the process of purification.

Answer 20

• plate A – Pure copper
• plate B – Impure copper
• the solution – Copper sulphate

Copper from impure copper plate is transferred to the pure copper plate by the process of electroplating.

Question 21

Observe the following circuit given in Fig. 14.9.

Current does not flow in the circuit if there is a gap between the two wires. Does it indicate that air is a poor conductor of electricity? Does air never conduct electricity? Explain.

Answer 21

Yes, air is a poor conductor of electricity. No, under certain conditions, such as during lightning, air may conduct electricity.

Question 22

 Boojho made the circuit shown in Fig. 14.10. He wanted to observe what happens when an electric current is passed through water. But he forgot to add a few drops of lemon juice to water. Will it make any difference to his observations? Explain.

Answer 22

If the water is distilled water and lemon juice is not added, no current will pass through the circuit. If the water taken is salty, then a feeble current will pass through the circuit and bubbles will be seen on the negative electrode.

Question 23

Observing that the bulb does not glow in the circuit shown in Fig. 14.11 A, Boojho changed the circuit as shown in Fig 14.11 B. He observed deflection in the magnetic compass.

• What does the deflection in magnetic compass indicate?
• Why did the bulb not glow in Fig.14.11 A?
• What would be the effect of increase in the number of turns in the coil wound around the magnetic compass in Fig. 14.11B?
• What will be observed if the number of cells are increased in the circuit shown in Fig. 14.11B?

Answer 23

• It indicates the presence of current in the circuit.
• The bulb did not glow because the current was not sufficient to make it glow.
• Deflection in the magnetic compass will increase.
• Deflection in the compass will increase further

 

Question 24

You are provided with a magnetic compass, an empty match box, a battery of two cells and connecting wires. Using these objects how will you make a tester for testing an electric circuit? Draw the necessary circuit diagram and explain.

Answer 24

Whenever current flows through the circuit the megnetic compass needle shows deflection due to magnetic effect of current.